LED6000 datasheet(39/49 Pages) STMICROELECTRONICS

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LED6000

Application notes - alternative topologies

The main drawback of this topology, compared to inverting buck-boost, is the slightly lower

efficiency. In the positive buck-boost Equation 27 on page 35 is not applied, so the

maximum input voltage is the maximum operating input voltage for the LED6000, as shown

in Table 5 on page 8. In addition, the maximum output voltage can be higher than 61 V,

assuming that the external MOS FET and Schottky diode are properly rated.

The other considerations summarized in Section 6.1 on page 35 are also applied to this

topology.

Figure 27 shows the schematic circuit for an LED current source based on positive buck-

boost topology. The input voltage ranges from 24 to 54 V and it can drive a string composed

of 14 LEDs with 0.55 A DC.

Figure 27. Positive buck-boost schematic

This schematic also includes two additional circuitry:



A level shifter / clamping network for safe MOS driving and protection;



An external overvoltage protection which avoids the IC damage in case of the LED

open row fault. This function is implemented by a Zener diode, D3, R7 and the sensing

resistor, RSNS = R13.

In case the LED string is disconnected the maximum output voltage is given by:

Equation 33

MAX

OUT





LED6000 数据表(PDF) 39 Page - STMicroelectronics